DNA damage checkpoints are critical for the maintenance of genomic stability. Many proteins involved in cell cycle checkpoints contain functional domains such as forkhead homology-associated (FHA) domains and BRCA1 C-terminal (BRCT) domains. Although these domains are believed to be involved in protein-protein interaction, the exact roles of these domains in DNA damage pathways are not yet clear. We have recently studied a previously uncharacterized FHA/BRCT-domain-containing protein Kiaa0170. We have shown that Kiaa0170 is an essential component in the DNA damage pathway and is required for all known ATM-dependent checkpoint and repair functions. Because of the critical role of Kiaa0170 in mediating DNA damage signal transduction, this protein was renamed as mediator of DNA damage checkpoint protein 1 (MDC1) to better reflect its function in vivo. To expand our knowledge of the roles of MDC1 in DNA damage-signaling pathways, we generated MDC1-deficient mice. The initial characterization of these mice and the subsequent mechanistic studies have led us to propose a working hypothesis for MDC1 function in the DNA damage response. We propose that the main function of MDC1 is to bridge an interaction between ATM and phospho-H2AX. In this proposal, we will further study the regulation and functional significance of the ATM/MDC1 interaction in Specific Aim 1. In Specific Aim 2, we will explore whether and how the loss of MDC1 contributes to tumorigenesis. Interestingly, we have also observed cellular senescence and premature aging related phenotypes in MDC1- deficient cells and mice. Thus, in Specific Aim 3, we will study how the loss of MDC1 leads to cellular senescence and aging-related phenotypes in vivo. Collectively, these studies will provide insights into the function of MDC1 in DNA damage pathways and the molecular mechanisms underlying the DNA damage signal transduction in mammalian cells.